1. Field of the Invention
The present invention relates to an optical modulator for use in optical communication and optical measurement fields.
2. Description of the Related Art
With the spread of the Internet, communication traffic is increasing dramatically, and optical fiber communication is becoming very important. In optical fiber communication, electric signals are converted into optical signals, and the optical signals are transmitted through optical fibers. Optical fiber communication is characterized by broadband, low-loss signals with high resistance to noise.
Electric signals are converted into optical signals by direct modulation using a semiconductor laser or external modulation using an optical modulator. Although the direct modulation needs no optical modulator and is inexpensive, the direct modulation has a limited modulation rate. Thus, external modulation is used in high-speed long-distance applications.
In practically used optical modulators, an optical waveguide is formed by titanium (Ti) diffusion in the vicinity of a surface of a single-crystal lithium niobate substrate. High-speed optical modulators of 40 Gb/s or more are commercially available. However, these high-speed optical modulators have the drawback of having a length as long as approximately 10 cm.
Japanese Unexamined Patent Application Publication No. 2006-195383 discloses a Mach-Zehnder optical modulator that includes a c-axis oriented lithium niobate film as an optical waveguide. The lithium niobate film is formed by epitaxial growth on a single-crystal sapphire substrate.
However, this optical modulator including the lithium niobate film has the following problems. FIG. 15 is a cross-sectional view of an optical modulator 100 described in Japanese Unexamined Patent Application Publication No. 2006-195383. A lithium niobate film is formed by epitaxial growth on a sapphire substrate 21, and optical waveguides 22a and 22b each having a rectangular cross section are formed by fine patterning. The side surfaces and top surface of the optical waveguides 22a and 22b are surrounded by a SiO2 buffer layer 23 (buried optical waveguides 22a and 22b). Electrodes 24a and 24b are disposed on the buffer layer 23 above the optical waveguides 22a and 22b. The SiO2 buffer layer 23 has a relative dielectric constant of 4, which is much lower than the relative dielectric constant of 28 (parallel to the c-axis) or 43 (perpendicular to the c-axis) of the lithium niobate film. Thus, even when a voltage is applied between the electrodes 24a and 24b, a sufficient electric field is not formed in the optical waveguides 22a and 22b. This causes the problem of a high half-wave voltage Vπ. Although the half-wave voltage Vπ can be lowered by increasing the electrode length L, this results in an increased size. Thus, the optical modulator has the problem of high VπL. Although the electric field applied to the optical waveguides 22a and 22b can be strengthened by using a material having a high relative dielectric constant for the buffer layer 23, this causes another problem of a low characteristic impedance Zc of the electrodes. Although the gap between the electrodes must be increased in order to maintain the characteristic impedance Zc, this weakens the electric field applied to the optical waveguides 22a and 22b. 